Dose-volume parameter evaluation of a sub-fractionation workflow for adaptive radiotherapy of prostate cancer patients on a 1.5 T magnetic resonance imaging radiotherapy system

Background and purpose:

This study focuses on evaluating a sub-fractionation workflow for intrafraction motion mitigation of prostate cancer patients on a 1.5 T magnetic resonance imaging radiotherapy system.

Materials and methods:

The investigated workflow consisted of two sub-fractions where intrafraction drift correction steps were applied based on a daily reference plan. However, the daily contours were only rigidly shifted to match the intrafraction anatomies and therefore the clinical dosimetric constraints might be violated. In this work, daily contours were deformed to match the intrafraction anatomies and the online plans were re-calculated for a total of 15 patients. The deformed prostate contours were inspected by radiation oncologists and corrections were performed when necessary. Finally, a dose-volume parameter evaluation was performed on a sub-fraction level using the clinical plan parameters.

Results:

Clinically acceptable coverage was reported for the target structures resulting in mean V_95% of 99.7 % and 97.8 % for the clinical target volume (CTV) and planning target volume (PTV) respectively. Sub-fractions with insufficient CTV dose can be explained by the presence of intrafraction rotations and deformations that were not taken into account during intrafraction corrections. Additionally, for no sub-fraction the dose to the organs-at-risk exceeded the clinical constraints.

Conclusion:

Given our results on the CTV coverage we can conclude that the sub-fractionation workflow met the dosimetric constraints for the hypofractionated treatment of the analyzed group of prostate cancer patients. A future dose accumulation study can provide further insights into the suitability of the clinical margins.